Woodbase material boards of this kind are widespread and are used in many different fields. One particularly large field of use is their application as flooring panels. In the context of this use in particular, the woodbase material boards provided with the decoration are subject to high loading. In order that they withstand these loadings, the decorative layer must be covered with a protective layer. The latter consists generally of a synthetic resin, such as melamine resin, to which various adjuvants have been admixed. As a result of the different layers applied to the woodbase material board, there are tensile stresses, which can lead to dishing of the woodbase material board. For this reason it is necessary for both the top and bottom faces of the woodbase material board to be coated, in order to allow these forces to occur evenly on both sides and thus to prevent dishing.
EP 2 338 693 B1 discloses the application, to the top face and/or bottom face of the woodbase material board bearing the decoration, of a first upper resin layer which comprises corundum particles, the drying of this upper resin layer, and then the application of a second resin layer which comprises cellulose. The second resin layer is then dried, and atop this second resin layer a third resin layer is applied, which comprises glass particles and is dried.
EP 1 339 545 B1 discloses an antiwear layer based on synthetic resin that can be used to protect from wear the surfaces of furniture or of floors, consisting of a decorative paper and optionally further papers lying above the decorative paper. This antiwear layer comprises resin material particles having a Mohs hardness of at least 6, and further compact, round particulate solids substantially free from cut edges, these particulate solids being in the form of beads having a Mohs hardness of at least 5. The latter beads may be glass beads.
EP 1 512 468 B1 describes a method for sealing a construction board made from wood or from a woodbase material. There, first of all, liquid resin is applied to the top face and is then dried. The construction board is then compressed under the effect of temperature, with the resin undergoing at least partial melting. Additionally, according to EP 2 098 304 A2, a relief may also be embossed into the melting resin, corresponding to the decoration on the top face of the woodbase material board.
In all of the known methods, a foamed layer of melamine resin and cellulose fibers is applied to the large-format woodbase material board, coated directly with a decoration, and following application this foamed layer is actively dried in order to protect the decoration and so to allow the board to be stacked and stored. A board coated directly with a decoration means a board whose top face is imprinted with a decoration in one or more plies; in other words, no decorative paper is used. The decoration may be printed directly onto the top face of the woodbase material board, or a priming coat may be provided between the top face and the decorative layer.
A customary layer system on the top face of a woodbase material board consists of 15 to 40 g/m2 of primer, which consists of an aqueous melamine resin. Applied atop this primer layer, as a white base, is an aqueous white paint in an amount of 20 to 30 g/m2. The decoration consists customarily of two, three, or four decorative prints applied to the white base. This decoration is then covered with a covering varnish which consists of an aqueous melamine resin/cellulose fiber mixture, which is foamed immediately prior to application. The covering varnish is applied in an amount of 10 to 15 g/m2.
The covering varnish is necessary in order to protect the decoration in subsequent operations and coating of the woodbase material board in the downstream manufacturing operation. The boards are not necessarily processed to completion in a continuous procedure, but instead are entirely likely to be put in interim storage, and stacked with one another for that purpose. The covering varnish is necessary, consequently, but is particularly critical in its application, having consequences, in particular, in downstream operations, resulting in product-critical error patterns through to rejection. The range here lies between a melamine resin/cellulose fiber mixture with too little foaming, which can lead to instances of sticking between decorated woodbase material boards within a stack, and an excessively foamed melamine resin/cellulose fiber mixture, which can lead to the graying of the decoration, something which does not become apparent, however, until after lamination in a short-cycle press.
Various attempts to adjust the foaming and to keep it constant have failed, since the processing operation divides up in terms of time and physical location, thereby making a regulatory intervention impossible or possible only at unacceptably high levels of cost and inconvenience.